Ceramic materials are used for a variety of purposes including, for example, in the manufacture of heat-resistant substrates and insulating substrates. The ceramic material can be shaped and compressed under pressure or laminated onto another substrate using known techniques. Common ceramic materials include, for example, barium titanate, strontium titanate, lead titanate-lead zirconate, ferritic ceramics, and glass ceramics. The ceramic material is shaped into the desired form using various metal molding techniques, such as, compacting, tape or ribbon casting, extrusion molding and injection molding. The shaped ceramic material is then heated at high temperatures to sinter the ceramic material and form the finished ceramic substrate.
Ceramic materials are also used in forming trays and other devices for supporting a ceramic or powdered metal compact during the firing process to form a sintered ceramic or metal product. Powder metallurgy is a common and economical process for producing complex molded metal parts with high quality and high accuracy. The metal products are made in a similar manner as ceramic products by shaping the metal powder by pressure molding, extrusion or injection molding and compressing the metal powder to the desired shape. Thereafter, the compressed metal powder is baked or fired at a high temperature to sinter the particles and form the molded article. During the firing process, the article is supported on kiln furniture such as a ceramic fixture, sagger, plate or other support surface. Ceramic materials for the kiln furniture are particularly desirable due to the heat resistance and ease of manufacture.
The kiln furniture for supporting the metal or ceramic part during the firing process is made of a refractory material. Examples of ceramic materials include alumina with a bulk specific gravity of 3.0-3.3, alumina-silica with a bulk specific gravity of 2.5-3.0, zirconia with a bulk specific gravity of 4.0-5.0, and magnesia with a bulk specific gravity of 2.5-3.0. In recent years, various lightweight devices have been used to reduce the heat energy required during the firing of the metal or ceramic part. For example, lightweight devices have been made from alumina or alumina-silica compounds. Lightweight kiln furniture has also been manufactured from a ceramic fiberboard made from a heat-resistant inorganic fiber, an inorganic binding material and a heat-resistant powder. Further examples of lightweight devices have been made from ceramic foams made from a sponge-like urethane foam impregnated with a slurry of a powdered ceramic material which is then heated to form a porous ceramic structure. Other lightweight ceramic kiln furniture is made from a refractory material such as an alumina-silica ceramic or a ceramic fiberboard material which is then coated with a thin layer of zirconia.
Porous ceramic substrates have also been produced by forming a mixture of polymeric thermoplastic beads and a ceramic material and heating the mixture to decompose the polymeric thermoplastic beads leaving a porous ceramic body. The resulting ceramic body has interconnected spherical pores and a high strength and thermal shock resistance. An example of this type of ceramic material is disclosed in Japanese Patent Publication 3-1090. The ceramic body is generally formed by combining a granulated ceramic material with a polymeric thermoplastic bead, granulating and mixing the ceramic material and polymeric thermoplastic uniformly. The mixture is then compressed by press molding, removing the polymeric beads from the green part and firing the resulting part to form the finished product. In this process, the granulated ceramic and polymeric thermoplastic beads are mixed uniformly to avoid variation in strength in the products or portions of the resulting product. Uniform mixing of the ceramic material and the polymeric thermoplastic beads prevent separation of the ceramic granules from the polymeric beads.
This prior process, however, has the disadvantage of encapsulating the polymeric beads in the ceramic granules due to the intimate mixing of the ceramic and polymeric beads. The finished product has a surface completely covered with a powder of the ceramic material which completely seals the polymeric beads within the ceramic. During the heating process, the encapsulated polymeric beads within the ceramic powder form cracks in the ceramic. In extreme cases, the encapsulated polymeric beads can create cavities inside the part as a result of the internal pressure of the gas being generated by the heating and decomposition of the polymeric beads during the heating process. Consequently, these prior processes make it difficult to produce a uniform ceramic body having a uniform strength and surface texture.
To overcome the disadvantages of the prior processes which encapsulate the polymeric beads, other methods were developed to obtain a stable product and uniform ceramic body with good strength. In this process, the polymeric beads used for forming the pores and the ceramic component as the raw material to make the ceramic body have the same particle size distribution. The difference in the particle shape and the particle size between the polymeric beads and the ceramic granule is small. Coordinating the particle size distribution of the polymeric beads and the ceramic component during the mixing of the base materials prevent the mixture from separating during the molding process. In addition, the polymeric beads are easily removed during the heating step. An example of the process is disclosed in Japanese Patent Publication 7-223879.
There has been a recent increase in the demand for smaller and more accurate ceramic parts and metal parts made by the powder metallurgy processes where the shaped product is fired to produce the finished part. The finished part is produced by producing a green part from a powdered composition and firing the green part at a high temperature. During the firing process, the size of the part can shrink by as much as 20%. In order to obtain a highly accurate finished product, it is important that the part shrink uniformly in all dimensions. It is known that the properties of the raw material and the packing density of the part affect the uniform shrinking. In addition, it is recognized that the support surface of the kiln furniture supporting the part affects the finished product during the shrinking of the part. It is important to have a sufficiently smooth surface supporting the part which does not hinder the shrinking. It is desirable that the support surface have a substantially smooth surface and a low friction coefficient.
When heating relatively large green parts on kiln furniture which has a rough surface, the rough surface of the kiln furniture is transferred to the surface of the part during the heating and shrinking process. Typically, this produces a rough or streaked face on the surface of the resulting part. In other cases, particles from the kiln furniture can break off and adhere to the part being heated.
Efforts to produce kiln furniture having a smooth surface resulted in a lightweight fiberboard material made from a refractory composite material. The fiberboard can have a surface roughness of 2-40 microns as generally disclosed in Japanese Patent Publication 62-275078. Other processes have produced a lightweight fireproof material having a surface coated with Al.sub.2 O.sub.3 alumina powder having a 95-100% purity. This produces a composite material having a surface roughness of 2-40 microns as disclosed in Japanese Patent Publication 62-283885. These fiberboard materials, although being refractory, do not have sufficient strength and are subject to scratching. The soft surface of the fiberboard makes it difficult to maintain a smooth surface, thereby increasing the maintenance of the face of the fiberboard.
Accordingly, there is a continuing need in the industry for ceramic materials and kiln furniture which overcome the disadvantages of the prior ceramic materials.